Innate immunomodulation with BCG in porcine monocytes enhances responsiveness to heterologous agonists

Immunomodulation engages the host’s own immune system to fight against disease. The innate immune system recognizes and responds to a large range of foreign agonists, while the adaptive immune system recognizes very specific antigens. The vaccine strain of Mycobacterium bovis (Bacillus Calmette-Guerin; BCG) induces an alternative phenotype in monocytes that enhance their ability to respond to a range of heterologous agonists. This alternative phenotype is a form of innate memory and is characterized by long lasting epigenetic and metabolic changes in innate cells, such as monocytes and NK cells, that enhance the responsiveness of these cells to future heterologous agonists. As a method to improve animal health and reduce the use of antibiotics, we interrogated the ability of BCG to alter the innate memory phenotype in pig monocytes. Primary porcine monocytes were stimulated with either live or inactivated BCG (Danish strain). After 24h the supernatants were collected and the cells maintained in culture for 5d. Cells were restimulated with the heterologous agonist lipopolysaccharide (LPS; TLR4 agonist) or Pam3CSK4 (synthetic triacylated lipopeptide; TLR 2 agonist). Cells and supernatants were collected after restimulation for gene expression and cytokine production analysis. Enhanced innate memory (trained immunity) is characterized by increased cytokine production relative to non-stimulated controls. Priming with either live or inactivated BCG enhanced IL-1β and TNFα cytokine production when restimulated with either LPS or Pam3CSK4. Monocytes primed with BCG, but not restimulated with a heterologous agonist, did not produce measurable amounts of cytokine, indicating that the increased cytokine production observed with LPS or Pam3CSK4 restimulation was not due to residual cytokine production from the primary BCG stimulation. No significant differences were observed in IL1B or TNFA gene expression 6h after restimulation with LPS, but live BCG priming upregulated expression of caspase-1 and NLRP3 to LPS restimulation. Collectively, BCG can alter the monocyte innate memory phenotype and enhance responses to heterologous agonists. Innate memory may serve as a mechanism to enhance immune responses and reduce the use of antibiotics.
 
Keywords: innate training, immunomodulation, pig, monocyte, heterologous protection.

Abstract presented at the 3rd International Symposium on Alternatives to Antibiotics 2019.